Study introduces new nanoscale vacuum channel transistors

by Ingrid Fadelli , Phys.org

Electron emission trajectory through the vacuum transistor from the source (bottom) to the drain (top). Credit: Jin-Woo Han.

Vacuum tubes initially played a central role in the development of electronic devices. A few decades ago, however, researchers started replacing them with semiconductor transistors, small electronic components that can be used both as amplifiers and switches.

Although vacuum tubes are now rarely used in the development of electronics, they have several important advantages over transistors. For instance, they typically enable faster operation, better noise immunity and greater stability in extreme or harsh environments.

In a recent study, researchers at the NASA Ames Research Center have demonstrated that nanoscale vacuum channel transistors can be fabricated on silicon carbide wafers. Fabricating this type of transistor on the wafer scale could ultimately enable their widespread use, making them a viable alternative to solid-state electronics.

"Off-the-shelf-electronics have very little use for space missions because of the impact of radiation," Meyya Meyyappan, one of the researchers who carried out the study, told TechXplore. "Typically, radiation shielding or advanced radiation-aware circuit design would be needed, all of which are expensive, time consuming and result in hardware that is not the state-of-the-art. We have combined the best of vacuum physics and modern integrated circuit manufacturing to produce nanoscale vacuum transistors to overcome the above shortcomings."

When fabricating the nanoscale vacuum channel transistor, Jinwoo Han, the researcher responsible for the design and fabrication, followed a similar process to that employed when building conventional MOSFETs (metal oxide semiconductor field-effect transistors). The only difference was that he replaced the semiconductor channel, which in MOSFETs is placed between the source and the drain, with an empty channel.

"Unlike our earlier works on silicon surround gate nano vacuum transistors, we have flipped the orientation this time to vertical instead of a horizontal transistor," Meyyappan explained. "Since the channel has nothing, electrons can be faster than in semiconductors where they experience scattering with the lattice, and thus the operating frequency or speed can be higher."

The nanoscale vacuum channel transistor presented by the research was fabricated on 150mm silicon carbide wafers. When evaluating its performance, the researchers found that the drive current of their transistor scales linearly with the number of emitters on the source pad.

Meyyappan and his colleagues also compared its performance with that achieved by silicon vacuum channel transistors fabricated simultaneously. Their tests revealed that the silicon carbide device offers significantly superior long-term stability, which could be particularly beneficial for applications in space and in other challenging environments.

"We have fabricated our sub-100 nm feature scale vacuum channel transistors in both silicon and silicon carbide material systems," Han told TechXplore. "Their performance is encouraging and the transistors are not affected by radiation. The implication is that we can use our current manufacturing infrastructure and known material systems to make ultrasmall vacuum devices."

In the future, the findings gathered by Meyyappan, Han and their colleagues could promote the reintroduction of vacuum channel transistors for the fabrication of electronics, particularly for those designed to be used in space. Meanwhile, the researchers are planning to use the transistors they developed to build circuits, in order to apply them and test them in real-life settings.

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Silicon carbide can handle very high temperatures, so SiC transistors would be very good in high temperature environments. So besides avoiding radiation issues, these devices may reduce or eliminate the need for cooling in hot environments or when handling heavy loads.

So basically the gate surrounds the channel, with nothing in the channel (except current). I wonder what the range of gate sensitivity is? Seems like it would vary with the width of the channel. And how much gate current is required? It should be very small. Also, although the channel is vacuum, the source and drain presumably aren't. So there's a limitation there.

But not bad, not bad at all; I don't know the heat dissipation pattern of a MOSFET, whether most of the heat is dissipated in the channel or in the source and drain.

In a MOSFET, the channel is the switch, so I'd expect that in the on state, the low resistance of the channel would be the place that heats as the voltage drops between source and drain. I have no idea how that works with a vacuum as the channel... At high frequencies, capacitive effects become a source of energy loss. Wonder how vacuum SiC compares to conventional semiconductors?

At least there's finally a solution for playing back the Voyager records with that classic tube sound... :P

Just off the top of my head, I'd expect the most heat in the channel since it's the smallest, which means that not only is the main heat source quenched, but using SiC the heat tolerance is much higher. I'd like to see heat profiles and the gate currents to be sure.

I should also point out, @carbon, that the channel is half the switch; the gate is the other half. Keep in mind as well that there won't be much voltage drop across a vacuum.

Capacitance across a vacuum is less than across any semiconductor by definition. The real question is how long the source and drain take to charge and discharge; that will determine the capacitance in this case.

And one last point: this article is about using these transistors rail-to-rail, not in the active region for more than the instant it takes to switch-on-switch-off. I would be interested to see how these devices behave in the active region, that is, when they are used (per the article) as amplifiers rather than switches. This will determine how useful they will be in switching power supplies, whose inefficiencies are mostly in the brief intervals while switching.

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Study introduces new nanoscale vacuum channel transistors

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